The present invention relates to an acquisition/distribution layer and a method of making the same, and more particularly to such an acquisition/distribution layer which is carded, chemically bonded, and thermally bonded.
Acquisition/distribution (A/D) layers are well known in the diaper art. They should be bulky, soft, and three dimensional to promote bodily fluid distribution over much of the layer""s outer surface (adjacent to the cover sheet, which in turn is adjacent to the wearer) before there is substantial absorption of the fluid by the absorbent core located on the other side of the A/D layer. Otherwise the fluid would overwhelm the absorption material at the relatively narrow point of absorption rather than being spread over a wide area of the absorption material surface for superior absorption. Such A/D layers have been made in the past by a variety of different processes as follows:
1. Chemical Bonding. A bale of fibers was opened and carded into a nonwoven web. The nonwoven web was then mixed, at a relatively slow rate of less than 70 meters/minute, with over 25% by weight of a latex binder (including water and expensive latex (rubber) powder), and then the mixture was heated both to chemically cure the latex on the nonwoven and to evaporate the water. This was an expensive, slow and environmentally unfriendly process. The resulting product was stiff and flat as well as expensive due to the high level of latex binder required and the high level of water to be evaporated.
2. Thermal Bonding or xe2x80x9cThermobonding.xe2x80x9d Bicomponent fibers were incorporated into a carded web at a level of at least 25% and then the carded web was heated to make a thermobonded fabric. While such thermobonded products were bulkier and softer than the chemically bonded nonwovens and could be produced at higher speeds than the chemically bonded materials, the need to use expensive bicomponent fibers increased the cost of such product.
Accordingly, it is an object of the present invention to provide a process which utilizes a newly developed high speed carding process.
Another object is to provide such a process which utilizes a faster and more environmentally friendly chemical bonding process which employs a lower level of latex binder more efficiently and requires less water to be evaporated.
A further object is to provide such a process which utilizes a thermal bonding process which uses a lower level of bicomponent fibers.
It is also an object of the present invention to provide an acquisition/distribution layer which is inexpensive relative to those formed exclusively by chemical bonding or thermal bonding.
It is another object to provide such an A/D layer which is soft, bulky, environmentally friendly, and producible at higher speeds than prior art A/D layers.
It has now been found that the above and related objects of the present invention are obtained in a method of manufacturing a nonwoven fabric comprising the step of forming a supported carded web. The web includes 75-89% by weight, based on the carded web, of first monocomponent fibers having a first denier of 3 to 7, 10-20% of second monocomponent fibers having a second denier of 8 to 20, the second denier being substantially higher than the first denier, and 1-5% of bicomponent fibers having a denier generally similar to the first denier. The supported carded web is heated to soften one component of the bicomponent fibers. The heated supported carded web is cooled to cause the softened one component to bind together the first, second and bicomponent fibers to form a self-sustaining pre-bonded web. Six-10% by weight latex particles, based on the total fiber of the self-sustaining pre-bonded web, are deposited in the interior and on the exterior of the self-sustaining pre-bonded webxe2x80x94e.g., by passage of the self-sustaining pre-bonded web through a saturation bath. Concurrently, the associated latex particles are smeared over the exterior of the self-sustaining pre-bonded web and the associated latex particles in the interior and on the exterior of the self-sustaining pre-bonded web are curedxe2x80x94e.g., by passage of the self-sustaining pre-bonded web and associated latex particles over heated rollsxe2x80x94to form a bonded web of acceptable tensile strength. Excess moisture is removed from the bonded web to form the nonwoven fabric.
In a preferred embodiment, the first monocomponent fibers are polyester, polyethylene or polypropylene of 3 to 7 denier (preferably 5-6 denier), and the second monocomponent fibers are polyester, polyethylene or polypropylene of 8-20 denier (preferably 10-12 denier). The supported carded web is heated by infrared radiation, preferably to a temperature of at least 170xc2x0 C. The heated supported carded web is then cooled to a temperature below the softening point of the one component so that the one component of the bicomponent fibers bonds together the first fibers, the second fibers and the other component of the bicomponent fibers. The saturation bath through which the pre-bonded web passes is at a temperature of at least 25xc2x0 C., and is an emulsion preferably containing at least 8% by weight latex particles. The heated rolls over which the self-sustaining pre-bonded web and associated latex particles are passed over are at least eight steam-heated ceramic rolls at temperatures of from 85xc2x0 C. up to 190xc2x0 C. and exert a pressure of about 20 psi on the self-sustaining pre-bonded web. The excess moisture is removed from the bonded web by passage thereof through a hot-air oven at a temperature of at least 200xc2x0 C. to leave the bonded web with a moisture level of less than 10%. The bonded web has a tensile strength of a least 800 grams/inch in MD and at least 60 grams/inch in CD.
The present invention also encompasses a nonwoven fabric comprising a self-sustaining pre-bonded web formed by carding together 75-89% by weight, based on the total fiber, of first monocomponent fibers having a first denier of 5 to 6, 10-20% of second monocomponent fibers having a second denier of 10 to 12, the second denier being substantially thicker than the first denier, and 1-5% of bicomponent fibers having a denier generally similar to the first denier. The bicomponent fibers have a first component with a low softening point and a second component with a relatively higher softening point, the first component binding together the first, second and bicomponent fibers to form the self-sustaining pre-bonded web. Six to ten percent by weight, based on the total fiber of the self-sustaining pre-bonded web, cured latex particles are disposed within the fabric and smeared on the outer surfaces of the fabric, the particles being cured in situ to provide enhanced tensile strength to the fabric.
In a preferred embodiment, the first and second monocomponent fibers are, independently, polyester, polyethylene or polypropylene, preferably polyester. The first component of the bicomponent fibers bonds together the first fibers, the second fibers and the second component of the bicomponent fibers. The fabric has a moisture level of less than 10% and a tensile strength of at least 800 grams/inch in MD and at least 60 grams/inch in CD.